Global Geodetic Observing SystemThe Global Geodetic Observing System (GGOS) has been established by the Int national Association of Geodesy (IAG) in order to integrate the three fundamental areas of geodesy, so as to monitor geodetic parameters and their temporal varia ? 9 tions, in a global reference frame with a target relative accuracy of 10 or b ter. These areas, often called pillars, deal with the determination and evolution of (a) the Earths geometry (topography, bathymetry,